Autoimmunity is thought to result from a combination of genetic susceptibility (what you inherit from your parents), breakdown in natural tolerance mechanisms (how your body decides what is harmful and what is not), and environmental triggers such as infections (that confuse your body about what is good or bad).

So we don’t really know what exactly causes it and hence we can’t really point out why once you get an autoimmune disease you are more likely to get others.

In some individuals, mechanisms that prevent autoimmunity like the central tolerance mechanisms that eliminate newly formed strongly auto-reactive lymphocytes are impaired. (Your body allows lymphocytes that kill your cells to stay.) Obviously, you are more susceptible in such a case. In others, there are genetic risk factors like genetic variants of alleles of MHC class II molecules.